The present invention relates to polyorganosilsesquixanes with superior heat resistance, combustion resistance and flexibility, expressed by the following formula (1) having a highly regular ladder structure wherein two kinds of substituents are bonded alternately, and a preparation method thereof, 
wherein R1 and R2 represent a hydrogen atom, a lower alkyl group such as methyl, ethyl and propyl, an aromatic compound like phenyl, halogenated phenyl group (C6H5Cl, C6H5RCl), an allyl group or a cyclo-compound; and
wherein n represents an integer that is sufficiently high that said polyalkylaromaticsilsesquioxanes (1) is a solid at up to 110xc2x0 C.
A conventional polyorganosilsesquioxane was obtained by performing condensation polymerization of an oligomer by heating, which was obtained by hydrolyzing trichlorosilane or triethoxysilane in the presence of alkaline catalyst in highly viscous solvents like NMP, DMSO and MIBK [Brown et al., J. Am. Chem. Soc., 82, 6194 (1960)]. By this hydrolysis method, oligomers (Mn=1000-3000, Mw/Mn greater than 2) other than a precursor (silantriol) tend to be formed because a condensation occurs concurrently with a hydrolysis. And, a polymer obtained from this oligomer in the presence of alkaline catalyst has problems in that:
a. It usually has a large molecular weight distribution and low molecular weight (Mn below 20,000-30,000); and
b. It is insoluble in an organic solvent because a 3-dimensional network structure is formed during the condensation polymerization due to the defect and randomness of an oligomer structure.
Also, using this polymerization method it is almost impossible to control the molecular weight of polymers.
Another method of preparing a silicon ladder polymer using phenylsilantriol [L. J. Tyler et al., J. Am. Chem. Soc., 77, 770 (1955), T. Takiguchi et al., J. Am. Chem. Soc., 81, 2359 (1959), E. C. Lee et al., Polymer Journal, 29(8), 678 (1997)] enables a preparation of a highly regular polymer with 1,000-1,000,000 of average number molecular weight (Mn) and lower than 2 of molecular weight distribution (Mw/Mn). However, the preparation process becomes complicated to obtain silantriol in high purity and the treatment of phenylsilantriol is very difficult. Especially, the yield is very low (lower than 10-20%), so that it is not economical.
In order to solve these problems, the present invention aims at designing a molecular model wherein R,Rxe2x80x2xe2x80x94SiO3/2 can be regularly introduced into a main chain of a polymer in a simple way and a condensation method thereof.
The present invention uses 1,3-organodisiloxane expressed by formula (2) for the purpose of controlling a structure of a silicone ladder polymer, 
wherein X represents a hydrogen atom, Cl, OH, NH2 or COOH; and R represents a hydrogen atom, an alkyl group, an acetate group or a metal like Na and K.
In the present invention, 1,3-organodisiloxane is soluble in general organic solvents and the prepared polymer is also soluble in general organic solvent like aromatic hydrocarbons such as toluene, xylene, benzene and chlorobenzene; halogen hydrocarbons such as methlyene chloride, chloroform, dichloroethylene, trichloroethylene and trichloroethane; ethers such as THF, 1,4-dioxane, diethyl ether and dibutyl ether; ketones such as acetone, methyl ethyl ketone and methyl ether ketone; esters such as butyl acetate, ethyl acetate, ethyl acetate and methyl acetate; and dimethylformamide.
A concentration of 1,3-organodisiloxane is recommended to be in the range of 30-80 wt. %. Under the concentration below 30 wt. %, the condensation reaction is slow or insufficient; and under the concentration over 80 wt. % there is a tendency of gel formation.
Any catalyst may be selected to facilitate the condensation reaction of 1,3-organodisiloxane. However, alkaline metal hydroxides like NaOH, KOH and CsOH; amines like triethylamine, diethylene triamine, m-butylamine, p-dimethylamine ethanol and triethanolamine; quaternary ammonium salts; or fluorides are recommended. A concentration of the condensation catalyst is recommended to be 0.01-20 wt. % to 1,3-organodisiloxane.
In the present invention, the condensation reaction can be carried out by heating 1,3-organodisiloxane solution. A preferred reaction temperature is 50xc2x0 C.-350xc2x0 C., more preferably 100-150xc2x0 C.
A reaction time is 6-50 hr in case catalyst is used. If no catalyst is used, the reaction should be carried out for a long time at a high temperature.
If the 1,3-organodisiloxane is above 90% in purity, sufficiently high-molecular-weight polymer can be obtained by this method.
Hereunder is given a detailed description of the preparing method. However, it should not be construed as limiting the scope of the present invention.